1. Technical Field
This invention relates to a forms feed tractor and, more particularly to a continuous forms feed tractor for moving a web or record medium such as paper having edge perforations, through a printer, copier or other similar apparatus operating upon the record medium.
2. Background Art
Forms feed tractor mechanisms have been used to move continuous forms or computer paper through printers for many years. The paper utilized has pre-punched holes, called perforations, along both sides. A pair of oppositely mounted tractor mechanisms are provided in the paper handling portion of the apparatus. The pins of the tractor are inserted into the pre-punched holes, engage the paper and move it forward through the apparatus. Generally, the tractor pins are arranged on a belt in an endless loop with at least one sprocket means provided for rotating the endless loop thereby causing the pins and paper to move.
In one configuration of a prior art tractor, the belt is driven by the drive sprocket around a non-rotating tensioner or shoe. As the belt moves in a complex belt path around the shoe its effective chordal length changes. Chordal length is the distance between pins of the belt. This change in length is the result of varying tension in the belt. As the pins of the belt move through a range of positions due to the maximum and minimum tensioning, the chordal length and belt tension vary sinusoidally. This creates some undesirable conditions. Since chordal length and tension vary, high belt tension must be introduced to achieve accurate forms registration. As tension increases, the frictional forces between the belt and shoe increase necessitating an increase in the amount of torque required to drive the tractor. A further undesirable result is that a more powerful and hence more expensive motor is needed to drive the apparatus; otherwise, the paper will be improperly positioned in the printer or copier resulting in poor image formation. Moreover, the change in chordal length and tension mean that the belt is undergoing cyclical stress fluctuations which has a fatiguing effect on the belt. In addition, the belt is being fixed around the base of the drive teeth as it moves around the tensioner due to geometrical changes of chord length which also results in fatiguing the belt. Examples of tractors employing a non-rotating tensioner or shoe can be found in U.S. Pat. Nos. 4,199,091; 4,226,353; 4,453,660; 4,614,287; 4,614,508 and 4,707,158.
In other prior art tractors, either fixed center or spring loaded idler sprockets are used. With the fixed center sprocket, obtaining consistent belt tension is very difficult unless tolerances are tightly held which adds significant extra expense. If tolerances are not tightly held, belt tension will vary greatly causing inaccurate paper registration and poor image formation. In the case of the spring loaded sprocket, the magnitude and consistency of the belt tension is determined largely by the spring. Examples of tractors employing idler sprockets are described in U.S. Pat. Nos. 3,688,959; 3,825,162; 3,938,721; 4,130,230; 4,159,794 and 4,194,660.
Numerous attempts to overcome these difficulties have been attempted but with unsatisfactory results.